Basically, you can get more info on familytreeDNA.com, but in short Ia and I are the same. The difference is someone figured out that Hg I continues to exist and before it was just listed on the tree as Ia,Ib& Ic now it is I,Ib and Ic.
As far as M (Stanford Univ. Lab) #253 refers to the list of repeats of C G T A in the DNA. This being the 253 variant. Look on Wikipedia.com for more details.